Validation of in-vitro antifungal activity of the fermentation quotient on bread spoilage moulds through growth/no-growth modelling and bread baking trials

The main objectives were to investigate the antifungal effect of lactic (LA) and acetic acid (AA) and the fermentation quotient (FQ, mol/L ratio of LA over AA) towards Penicillium paneum and Aspergillus niger through growth/no-growth modelling of in-vitro data and to validate its effect in chemically acidified bread by performing challenge tests. Results have shown that AA has strong antifungal activity towards both bread moulds, albeit that P. paneum is ten times more resistant compared to A. niger. Complete growth inhibition of A. niger on chemically acidified bread was observed during 35 days at 21 °C for concentrations of AA ≥ 33 mmol/kg dough or ≥165 mmol undissociated AA/L water phase in bread. Whereas for A. niger the FQ must be close to zero and the concentration of AA in bread 33 mmol/kg dough to achieve complete growth inhibition, very high concentrations of AA and LA were needed to prevent growth of P. paneum (>100 mmol/kg dough). The acids also affected yeast activity in dough resulting in lower specific volumes and harder bread crumb. To conclude, the antifungal activity of the FQ cannot be generalized because its antifungal power depends on the sensitivity of the mould species towards LA and AA